The magnet was designed by Mr Jim Mitchell of ACT, Australia, who uses Dingham Autocouplers on his British outline Gauge O layout. Jim very kindly sent me a magnet for testing and gave his permission to reproduce the details here.
Jim's DIY magnet is of high resistance (about 18 ohms). It takes a relatively low current ( about 0.7A at 12V DC) so is unlikely to burn out and is also unlikely to burn switch contacts. Electrical supply and switch types similar to those recommended for the Dingham magnet would be suitable for the DIY magnet also.
The exact number of turns in the coil is not critical. You can either wind until the outside diameter is 16mm or until you have used 25g or 1oz of wire.
In Jim's design, the 3/8in nut serves only as a spacer so that the 5/16in bolt projects no more than 20mm from the coils. Jim fixes the magnets to the layout by self-tapping the bolt into a hole drilled through the baseboard.
In 4mm scale, a 5/16 bolt will not fit between the sleepers, so a 3/16 or 1/4in bolt should be substituted. Alternatively, a suitable nail, about the same diameter could be used.
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